1. Field of the Invention
The present invention relates to a method and system for measuring the quality of compressed video data and, in particular, to a method and system for assessing the objective quality of compressed pictures, namely Peak-Signal to Noise-Ratio (PSNR), without referring to the source video data.
2. Description of the Related Art
It is the ultimate goal of the video experts to provide most perceptually appealing video images to viewers. One way to determine whether the resulting image quality is good or poor is to ask a panel of viewers to watch certain video sequences to provide their opinions. Another way to analyze video sequences is to provide an automated mechanism to evaluate the excellence or the degradation of the video quality. This type of procedure is known as xe2x80x9cobjective video quality assessment.xe2x80x9d
A common approach for measuring the picture quality of a compression process is to make a comparison between the processed image and the unprocessed source images (hereinafter referred to as xe2x80x9cdouble-ended measurementxe2x80x9d). Various metrics are used, i.e., Block Artifact Metric (BAM), PSNR, Perceptually Weighted PSNR, etc., to evaluate an objective picture quality measure. In particular, the PSNR is most commonly used parameter for measuring any picture quality, especially in evaluating MPEG-2 video bitstream. However, the double-ended measurement has some drawbacks in that access to both the processed picture and the source picture is not feasible if the source data or the alignment between the source and processed pictures are unavailable. In order to overcome this problem, a xe2x80x9csingle-ended measurementxe2x80x9d has been proposed to monitor the video quality when the source is neither available nor controllable. Unlike the double-ended measurement, the single-ended measurement technique operates on the compressed picture without access to the source picture when evaluating video quality. Although there are various single-ended methods that have been proposed, no single-ended measurement uses the PSNR metric in evaluating the compressed pictures alone without access to the source picture or the compressed video bitstream. Accordingly, the present invention proposes an objective quality assessment using the PSNR metric to evaluate the objective quality of compressed pictures without utilizing the source data or the compressed video bitstream.
The present invention is directed to an apparatus and method for evaluating the quality of encoded video data without access to the source data or the compressed video bitstream, by utilizing quantization parameters estimated directly from the decompressed picture.
According to an aspect of the invention, the method of evaluating the picture quality includes the steps of: performing a discrete cosine transform (DCT) on at least a substantial portion of the decompressed video data to produce a set of DCT coefficients for at least one AC frequency band; simultaneously, extracting quantization matrix data for at least one AC frequency band and extracting a quantizer scale for each block of the decompressed video data; estimating a variance of the DCT coefficients; determining an average quantization error for each set of the DCT coefficients based on the resultant variance, quantization matrix, and quantizer scale; and, calculating a peak signal to noise ratio (PSNR) based on the average quantization error.
According to another aspect of the invention, the method includes the steps of: performing a DC computation operation to recover an intra-dc-precision level from the decompressed video data; and, if the intra-dc-precision level is less than a predefined threshold, classifying the decoded video data as an intra-coded; performing a discrete cosine transform (DCT) on the decompressed video data to produce a set of DCT coefficients for at least one AC frequency band; extracting quantization matrix data for the at least one AC frequency band; extracting a quantizer scale for each block of the decompressed video data; estimating a variance of the DCT coefficients; determining an average quantization error for each set of the DCT coefficients; and, calculating a peak signal to noise ratio (PSNR) based on the average quantization error.
According to anther aspect of the invention, the apparatus capable of evaluating the quality of encoded video data includes: a decoder for decoding at least a substantial portion of the encoded video data to produce decoded video data including a plurality of blocks; a discrete cosine transform (DCT) configured to transform the decompressed video data into a set of DCT coefficients for at least one AC frequency band; an extractor for extracting quantization matrix data for at least one AC frequency band and for extracting a quantizer scale for each block of the decompressed video data; a collector for estimating a variance of the DCT coefficients; a first calculator for determining an average quantization error for each set of the DCT coefficients based on the variance, the quantization matrix, and the quantizer scale; a second calculator for determining a peak signal to noise ratio (PSNR) based on the resultant average quantization error; and, a picture detector for detecting an intra-coded picture in each block of the decompressed video data. The picture detector further includes a means for performing a DC computation operation to recover intra-dc-precision level from the decompressed video data; and, a means for classifying the decoded video data as an intra-coded picture when the intra-dc-precision level is less than a predefined threshold.
According to another aspect of the invention, the apparatus includes: a decoder configured to decode compressed variable-length Huffman codes and for producing therefrom decoded data, and for extracting quantization matrix data and a quantizer scale for each block of the decoded video data; an inverse quantizer configured to perform inverse-quantizing of the decoded data output from the decoder to produce a set of DCT coefficients; an inverse DCT configured to transform values of pixels in blocks of signals output from the inverse quantizer to dequantize decoded data including the difference data; a motion compensation and adder for receiving reference data within the encoded video data and the difference data from said inverse DCT to form motion compensated pictures therefrom; a collector coupled to the output of the inverse quantizer for estimating a variance of the DCT coefficients; a first calculator for determining an average quantization error for each set of the DCT coefficients based on the variance, quantization matrix, and quantizer scale; and, a second calculator for determining a peak signal to noise ratio (PSNR) based on the resultant average quantization error. The apparatus further includes a video memory configured to store reproduced video data.
The foregoing and other features and advantages of the invention will be apparent from the following, more detailed description of preferred embodiments as illustrated in the accompanying drawings in which reference characters refer to the same parts throughout the various views. The drawings are not necessarily to scale; the emphasis instead is placed upon illustrating the principles of the invention.